In recent years, with an increase in the number of bits that an imaging device (image sensor) can process, high-dynamic-range (HDR) images have been produced. The dynamic range of images can be generally expressed by the ratio between the minimum luminance and the maximum luminance. HDR images have a contrast ratio (for example, 10000:1 or higher) between maximum and minimum lightness colors and can express the real-world scenes. HDR images can record all luminance values in the visible range and can support the dynamic range and color gamut equivalent to the human visual characteristics. HDR images provide advantages such as the abilities to express shades realistically, simulate exposures, and express glare.
For example, HDR content is provided in an ultra high definition Blu-ray (registered trademark) Disc (UHD-BD), a webcasting service such as OTT (over-the-top), and a digital broadcasting service.
Families who watch content have displays with different performance such as a display capable of displaying standard dynamic range (SDR) images of which the dynamic range is compressed approximately by 1/40 and an HDR-compatible display having the maximum luminance of 500 nit or 1000 nit. Thus, a process (hereinafter referred to also as “display mapping”) of adjusting the dynamic range of original content to the performance of an output destination display of the images is required (for example, see Patent Document 1).
In display mapping, if the dynamic range is converted just by linear scaling, a large amount of information may be lost, and for example, the appearance of a person may be greatly different from a source image to a result image. Such a loss of information is contrary to the intention of a content producer or supplier.
A graphics image such as menus or an on-screen display (OSD) operated by users is often displayed by being superimposed on a content screen. When a graphics image generated in SDR is combined with HDR content, if the graphic image is displayed as it is without luminance conversion, the graphic image appears dark and it becomes difficult to see the image. If the luminance of the graphic image is just linearly scaled, the graphic image appears too bright.